A general method of tracing the location of a radio beacon will be described hereinafter with reference to the accompanying drawings.
FIG. 1 is an exemplary view showing a typical radio beacon location computing system.
Referring to FIG. 1, a radio beacon 100 transmits signals in two or more frequencies f1 and f2, which are independent from each other. Then, at least three base stations 111, 112 and 113 receive the signals in the two frequencies, extract phase differences ΔΦ1, ΔΦ2 and ΔΦ3 based on a transmission distance in phase difference calculator 141, 142 and 143, and computes the location of the radio beacon 100 in the location computing server 120 based on the extracted phase differences to thereby compute and determine the location of the radio beacon 100.
Generally, the method that a radio beacon transmits signals in two or more frequencies and base stations receive the radio signals and compute a distance by calculating a phase difference based on a frequency interference phenomenon has a problem that the calculation for acquiring a distance between a base station and a radio beacon produces a plurality of solutions where the phase differences between the two frequencies are ΔΦ, 2π+ΔΦ, 4π+ΔΦ, . . . due to ambiguity of a phase repeating at a period of 2π.
Accordingly, the conventional radio beacon location tracing method using more than two different frequencies and a phase difference thereof has a limited coverage, which is an area where the phase difference between the two frequencies is smaller than 2π. Thus, the conventional method cannot be applied to an environment where the coverage is larger than the phase difference of the two frequencies, i.e., 2π.
Hereinafter, the conventional radio beacon location computing method using two frequencies will be described with reference to FIG. 2.
FIG. 2 is a diagram illustrating ambiguity in location computation (positioning ambiguity) caused by phase ambiguity.
One radio beacon (TS) 100 transmits radio signals by using two frequencies, and base stations RS1, RS2 and RS3 111, 112 and 113 covering the area where the radio beacon 100 is disposed measure the phase difference between the two frequencies and computes the distance to the radio beacon 100. The measured phase differences ΔΦ1, ΔΦ2 and ΔΦ3 correspond to distances R1, R2 and R3 210, 220 and 230, respectively. When circles are drawn by taking the distances as radiuses, an intersection 240 where the three circles meet is determined as the location of the radio beacon 100.
However, when it is assumed that only the base station RS1 111 has phase ambiguity, it is possible to predict that the radio beacon 100 exists at a location where the phase difference of the two frequencies is 2π+ΔΦ1. Thus, a circle having a distance R1 211 corresponding to 2π+ΔΦ1 as its radius can be drawn. This method yields a solution of another location 250 where circles having the distances R2 and R3 220 and 230 from the base station RS2 112 and the base station RS3 113 meet.
Therefore, there is a problem that the accurate location of the radio beacon 100 cannot be detected in an area where the phase difference between the two frequencies is larger than 2π.
To sum up, since the conventional location computing method using more than two frequencies and phase difference at a location where the frequencies arrive may produce a plurality of solutions due to the phase ambiguity, it should be used within an area where the phase difference between the two frequencies is less than 2π. The limitation in distance draws back the location computation of a radio beacon from enlarging into an area where the phase difference between the two frequencies is larger than 2π.